Osmium is a rare element as gold and silver and present at only approximately 4 ppb on the earth (Non-patent publication 1). Osmium is also considerably maldistributed (Non-patent publication 2). Its oxidation product osmium (VIII) oxide is used as an oxidant in organic synthesis, and for fixation and staining in sample preparation for electric microscopy, though it is rather expensive (Non-patent publication 3). For many commercial waste disposers it is often difficult to handle the osmium-containing waste liquid discharged from sample preparation for electric microscopy due to following reasons: osmium (IV) oxide contained in such waste liquid is not easily precipitated; the waste liquid contains organic materials such as body tissue; and a highly toxic osmium (VIII) oxide gas, whose tolerable concentration is as low as 0.002 mg/m3, could volatilize during the processing of the waste liquid. For these reasons, waste liquid has been stored in many research institutes. For instance, in Hamamatsu University School of Medicine approximately 100 L of waste liquid has been stored.
For a method of processing at laboratory level, for example, Non-patent publication 4 examines a method of solidification with tannic acid for storing, and Non-patent publication 5 examines a method of reduction and precipitation as metal osmium (0).
However, either method requires complicated operations for solidification of osmium, and thus processed osmium cannot be reused and has no other way than to be stored as solid waste.
For reutilization of osmium, Non-patent publication 6 describes a method in which osmium (IV) oxide is extracted from the waste liquid with carbon tetrachloride, adsorbed onto glass wall and removed to give osmium (IV) oxide, which is dissolved in water before use and is oxidized by the addition of hydrogen peroxide to give osmium (VIII) oxide, the later is to be reused. However, this method requires employment of toxic materials, and also requires complicated operations for solidification and use after solidification.
Non-patent publication 7 describes a method in which osmium waste liquid is reduced and resulted osmium (0) is separated by centrifugation, then the precipitate is dissolved in hydrogen peroxide solution, and potassium peroxydisulfate is added thereto, which is then oxidatively distilled on heating to generate 1% osmium (VIII) oxide solution. However, this method uses a wide variety and a large amount of reagents (e.g., to 450 ml of waste liquid, 54 g of potassium peroxydisulfate is added as an oxidant), and requires complicated operations ranging from reduction to oxidative distillation.
As described above, although osmium (VIII) oxide is an extremely important and rare material across many fields, it is generally stored in solidified form after use, and no effective method for reuse has been established so far.
On the other hand, ozone is widely used for its strong oxidizability in, such as, sterilization or antisepsis in effluent treatment or in deodorization of emission. Patent publication 1 describes a method for processing a solid surface by supplying the solid surface which contains either ruthenium or ruthenium oxide or osmium or osmium oxide with oxygen atom-donating gas such as ozone, generating a ruthenium compound or osmium compound having a high vapor pressure, and thereby etching the solid surface with it. However, there is no description about recovery of osmium (VIII) oxide in this publication.
In addition, Patent publication 2 which was published after the priority date of the present application describes a method of producing magnetic nanoparticle-fixed osmate (VI) salt, which can be recovered by bringing a magnet close to it after reaction and reused as a reusable fixed osmium oxide catalyst. However, this publication does not describe about recovering osmium (VIII) oxide itself.